a. Field of the Invention
This invention broadly relates to digital communications systems. More specifically, it concerns transmitting and receiving high speed digital information signals in binary form, such as computer data, over any noisy line medium, such as alternating current (AC) power lines.
b. Description of the Prior Art
Digital information, such as computer data, is known to be capable of transmission over existing alternating current (AC) power lines. The potential benefits of such data communication are well-recognized, including extreme versatility for interconnecting electronic office products, data terminals, remote printers, personal computers and the like. Creation of data paths may be accomplished simply by plugging the distributed terminals into any available AC outlet. Any type of equipment that can be run by a central computer could be linked to that computer through the same power cord already provided for such equipment. Such a central computer could be used to control various process equipment, including heating, lights, and air conditioning.
The presently common situation with the vast majority of users requiring digital communication lines is the use of hardwiring to interconnect the various components. This is expensive, inflexible, and generally provide higher data rates than are necessary for the average user. Since AC power wiring already exists in most, if not all, locations where data transmission is needed, reliable high-speed digital communication through this medium would provide significant cost savings and system flexibilty.
A widely available transmission medium, i.e., an AC power line in the frequency range of approximately 100 kiloHertz (kHz) to 500 kHz, generally exhibits unpredictable transmission characteristics such as extreme attenuation at certain frequencies, phase changes along the route, notches and discontinuities. Generally, three modes of noise most common: low voltage Gaussian noise, low voltage impulsive interference, and very high voltage spikes. Of these three, the low voltage impulsive interference tends to be the predominant source of data transmission errors, i.e., data transmission may be reliably accomplished even in the presence of Gaussian noise. As for high voltage spikes, they are relative infrequent and invariably cause data errors, with error detection/retransmission (ACK/NACK) being commonly recognized as the best method of recovering the lost information. Furthermore, these characteristics may vary significantly as load conditions on the line vary, e.g., a variety of other loads being added or removed from the current-carrying line. Such loads include industrial machines, the various electrical motors of numerous appliances, light dimmer circuits, heaters and battery chargers.
Past attempts to solve these problems have included a variety of single or multi-channel, narrow band transmission techniques. Narrow bandwidth, however, limits the data transmission capacity of the link. Furthermore, the changing noise environment on the AC power line significantly impairs the reliability of any technique which suffers when a transmission channel (a predetermined bandwidth) is interrupted or lost. For these and other reasons, AC power line communication has not in the past been regarded as either fast or reliable.
While multi-channel digital coding techniques have modestly improved the reliability and speed of power line communication systems, the cost of improvement has been bulky, sophisticated and expensive signal processing equipment. Thus the potential for power line data transmission has not yet been achieved, nor realistically even approached. For example, substantially error-free data transmission has been limited to data rates under ten kilobits per second (kbps). Even with such improved systems, reliability is highly suspect since any one or more of the predetermined narrow bandwidth transmission channels may suddenly become unusable, without warning, due to unpredictable variations in the power line transmission characteristics.
In recent years, data transmission over power lines has become significantly more difficult, due to changes in the nature of the distortion encountered. Due to the widespread use of personal computers and remote printers, the FCC issued regulations which place limits on conducted or radiated digital emissions from computing devices onto power lines. In order to satisfy these requirements, computer manufacturers routinely added filters appearing, from the line side, as very low impedances, such as very high capacitances having values on the order of 0.1 microfarad. This significantly affects distortion encountered by wideband signals, and at the same time can cause severe attenuation of certain narrow bandwidth signals.
Many common forms of carrier signal modulation have been attempted in connection with power line communication systems. In each of these schemes, the digital information is modulated onto a carrier and the carrier is then added to the AC power line. A receiver picks off the modulated carrier signal and then demodulates that signal to recover the digital data information. Two of the more common types are amplitude-shift keying (ASK) and frequency-shift keying (FSK). Both techniques have been generally regarded as being susceptible to electromagnetic interference (EMI) and radio frequency interference (RFI). A third principal modulation technique, phase-shift keying (PSK), has also generally been considered unsuitable because of increased susceptibility to noise interference and consequently fluctuating carrier signal attenuation.
In light of the aforementioned difficulties, power line communications has not been regarded as a potential local area network (LAN) medium, despite what should be a natural extension of LAN systems to an already existing data transfer medium reaching into virtually every office in a building, every home in a neighborhood, or anywhere else AC lines or other two-conductor media can reach. Instead, LANs are generally expensive hardwired installations delivering data transfer capacity far in excess of that required by most users (nodes) on the network.